1. Field of the Invention
The present invention relates to a thermionic emission cathode which is useful for electron beam apparatus.
2. Description of the Prior Art
Recently, in the field of electron beam apparatus such as scanning electron microscopes, electron beam processing apparatus and fine recording apparatus, it has been desired to have an electron beam source with a submicron diameter which also is very bright.
In general, the characteristics of thermionic emission are dependent upon the value of the work function of the particular material employed. The work function of tungsten which has been used in practice is 4.65 eV (sintered material), whereas the work function of lanthanum hexaboride is 2.66 eV (sintered material) which, of course, is smaller. This means that use of lanthanum hexaboride results in much emission current. Using a figure of merit equal to (work function)/Te (temperature required to provide 10.sup.-5 Torr of vapor pressure of the cathode material) as a basis for evaluation of a cathode, tungsten has a value of 1.6 .times. 10.sup.-3, lanthanum hexaboride has a value of 1.27 .times. 10.sup.-3 and Ba-O-Wa value of 0.95 - 1.05 .times. 10.sup.-3. A mono-atomic layer of Ba-O-W is superior to the others. However, it is difficult to maintain the surface of a cathode made of that material in the optimum condition. A lanthanum hexaboride cathode has been tested recently by cutting and processing a sintered lanthanum hexaboride in a desired size. An electron beam emitted from such a sintered lanthanum hexaboride cathode has been compared with that of a tungsten cathode and the advantageous characteristics thereof have been recognized. For example, the brightness using tungsten is 6.2 .times. 10.sup.4 A/cm.sup.2.str (2,500.degree. C) whereas the brightness using lanthanum hexaboride is 5 .times. 10.sup.5 A/cm.sup.2.str (12 KV at 1,700.degree. C) which is a high brightness. Moreover under the same brightness, the life of lantanum hexaboride is more than 100 times that of tungsten. Also the beam diameter provided by lanthanum hexaboride is several A which is similar to that of a field emission electron gun. A field emission type tungsten cathode gives a high brightness electron beam having a brightness of .about. 10.sup.9 A/cm.sup.2.str (100KW) in a very high vacuum such as .about. 10.sup.-9 Torr. However, when the high vacuum conditions are lowered, the emission current becomes quite unstable because of the effects of residual gas. Accordingly, the very high vacuum of .about. 10.sup.-9 Torr must be maintained in the apparatus using an electron beam.
This is a severe disadvantage for practical use.
For use in forming suitable structures for a sintered lanthanum hexaboride cathode, there have been proposed a method of heating it by direct passage of current using graphite as a cathode holder, and a method of heating one end of a sintered rod by radiant heat and an electron shock produced by a tungsten coil while holding the other end by a cooled copper block. Such structures are complicated compared with the conventional tungsten hair-pin type electron guns. Moreover, it is difficult to fix the sintered lanthanum hexaboride cathode on the holder of a scanning electron microscope or an electron microscope. Furthermore, it is preferred to reduce the curvature at the top of the cathode. However, the minimum curvature achievable has been about 10 .mu.m because of the necessity to shape it by mechanical grinding. Accordingly, it has been difficult to provide the desirable thermionic emission efficiency in the lanthanum hexaboride.